The objective of this study is to develop hydrophilic polymeric microspheres suitable to label membrane receptors and to separate cell subpopulations, using immunological techniques. The structure of the polymer and microspheres synthesized from monogluteraldehyde was elucidated by infrared (IR) and UVspectroscopy. Magnetite particles were incorporated into the polyglutaraldehyde (PGL) microspheres (1,000 Angstroms in diameter). The latter were covalently bonded to anti- bodies. IgG-bearing lymphocytes were labeled with the magnetic PGL microsphere-antibody conjugates and separated from unlabeled cells by a magnetic field. The electrophoretic mobility of human erythrocytes labeled with PGL microspheres was 40% less than that of unlabeled cells. This permitted separation of model cell mixtures by means of a free-flow electrophoretic apparatus. To\prepare immunoreagents in the form of microspheres of different sizes and morphologies, we polymerized acrolein in three ways: (1)\by ionizing radiation, (2)\by condensation polymerization in the presence of a base such as sodium hydroxide, and (3)\by polymerization in the presence of 4-vinyl pyridine. The main findings of our studies are as follows. (1)\Experimental conditions were established to prepare PA particles 100 Angstroms to 3 microns in diameter. (2)\Procedures were developed to produce intensely fluorescent PA microspheres of various sizes. (3)\A comparison of the Fourier IR analysis of PA prepared by base catalysis and PA obtained by Co gamma polymerization yielded evidence for a larger concentration of unreacted double bonds and a lower concentration of functional groups capable of reacting with amines in base-catalyzed PA than in PA prepared by ionizing radiation. This evidence was substantiated by nitrogen analysis and reactivity studies with antibodies. (4)\Small amounts of PA radiation grafted on commercial polymeric spheres could be detected with m-amino phenol, which rendered the surface of spheres intensely fluorescent. (5)\Using radioactive antibodies, the amount bound to PA prepared by two methods was determined in the pH range of 4 to 10. The optimum pH for maximum binding was on the order of 5. Neuroblastoma cells sensitized with monoclonal antibodies could be removed from bone marrow extracts by means of magnetic microspheres coated with sheep antimouse antibodies. Magnetic spheres coated with polyacrolein improve the efficiency of tumor cell removal and contribute to the stability of the magnetic spheres.